Prior art encoders use ten metallized pads delimiting a center portion containing a center electrode or inner ring made of metallized material. Each group of ten pads and inner ring or center conductor is called "dial." On electric meters, there are normally five dials. Such encoders use capacitive coupling to locate the position of the meter hands and a microprocessor associated with the encoder selects one pad at a time and one dial at a time. If the meter hand is over the selected pad, the pad/meter hand/inner ring capacitance is large. If the meter hand is not over the pad, the pad/inner ring capacitance is relatively small. This capacitance is measured and digitized and the digitized information is then processed by a microprocessor. The microprocessor compares the magnitude of the capacitance associated with each pad on a given dial. It locates the meter hand on the dial by finding the pad or two adjacent pads that have much larger capacitance than all of the other pads on the same dial and locates the meter hand on each dial by the same algorithm, or problem solving routine.
Reference is made to FIGS. 1 and 2. When the meter hand 9-10 is far away, (i.e., 25 mils) from the printed circuit board element 6, 7 and 8 of FIG. 1, the measured pad/inner ring capacitances will be on average much smaller if the meter hand had been very close to the board. If the meter hand is near the board the change in capacitances between the tail end and sides of the meter hand 9-10 and the pads 4-1 through 4-10 it is pointed to are much greater than when the meter hand is far away from the board. Consequently, all of the ten pads' responses must be compared with each other to determine the meter hand position.
When there are five dials involved, each of the like numbered pads of every dial is connected together. There are more than five dials on gas meter encoders. To select a dial, the dial's inner ring 2 is selected using one or more multiplexers. Each like number pad must be connected to circuitry to excite it with a set frequency signal. Multiplexers are used to route a Wien Bridge Oscillator 115 KHz to 175 Khz sine wave signal to the desired pad. Prior art encoders also use a squarewave generator made up of inverters and a resistor/capacitor combination, to generate a high frequency signal wave shaped to simulate the sine wave. Prior art wave shaping was performed by using a series resistor to each pad, high valued, precision, 1% tolerance resistors and the input capacitance of 10 microprocessor input ports for a 5 dial encoder. With this in mind it would be desirable to reduce the number of multiplexers required, the number of precision resistors and microprocessor input/output ports.
The present invention employs the technique of switch capacitance. This technique allows the number of pads to be reduced and to realize the savings that arise therefrom. Switch capacitance technique uses two CMOS analog switches per pad and two CMOS analog switches per inner ring or center electrode. In addition, it uses an equal number of AND Gates to properly Gate the clock signals that switch charges into and out of the pad and inner ring. Applicant has found that the output voltage varied in linear proportional manner with a change in capacitance between the selected pad and inner ring using this technique. Changes in capacitance were as small as 0.1 picofarads and were easily measured. When this signal was digitized, the linearity and proportionality was such that it could be preserved.
Digitized information of prior art encoders was used to determine a simple question: Is the meter hand over a particular pad or is it not? The decision was a binary one, having only two states or answers, yea or nay. Because each pad corresponded to one of the ten digits on a dial, this was an effective approach. However, the prior art did not address the situation where the pad did not correspond one for one with the ten digits on a dial. Prior art encoders use an eight bit analog to digital converter to achieve a required large degree of resolution. The prior art employs an analog to digital converter to convert the voltage, proportional to the pad inner ring capacitance, into an eight bit binary number, representing one of the 256 discreet voltage increments of this device. Consequently, there is an abundance of information available from a given dial that is thrown away by the prior art that could be used to determine not just whether the meter hand is over a selected pad, but where the meter hand is over a given pad.
Instead of using ten metallized pads and one inner ring (center electrode) per dial as prior art encoders, the present invention envisions a minimum of one or more electrodes or pads, a calibration ring and a center electrode or inner ring. By reducing the number of pads per dial the number of CMOS analog switches are reduced, a decoder is not needed and the number of input/output pins of a microprocessor are reduced. The present invention envisions a minimum of two pads (see FIG. 5) and one calibration ring. This requires only three microprocessor input/output pins to select a pad or calibration ring. The primary function of the center electrode or inner ring is considered a common point, selected when its respective dial must be read.
In prior art encoders, the distance between the meter hand and the printed circuit board is a variable distance. This is one of the major reasons that ten pads were used by the prior art. The capacitance formed between the pad/meter hands/inner ring is proportional to the distance between the board and the meter hand and the area of the pad(s) that the meter hand roughly shadows or covers. The shadow is similar to that cast by a human hand in diffused light. By holding one's hand just above a piece of paper, it can be noticed that a well defined shadow is formed on the page when the hand is close to the paper. As the hand is moved slowly away from the paper, the outlines of the hand get fuzzy and shadows more area of the paper. Capacitance between two electrodes is governed by the same principle. The more intense the shadow, the greater the capacitance. The fuzzier the shadow, the weaker the capacitance per unit area. By shadowing, it is not meant shadowing caused by light, but an electrical field created by exciting the pad and the inner ring electrode with a voltage signal or a voltage potential. Capacitance between the pad and the meter hand is defined approximately by the formula that C=e0*A/d. Wherein C is equal to capacitance, e0 equals permittivity of free space or air, and d equals the distance between electrode plates, namely the meter hand and metallized pad. This formula assumes that the plates are so close that there are very minor fringing capacitance effects, analog to a well defined shadow cast by a lighted object. As the plates move farther apart, the capacitance between them is found somewhere between the value of capacitance given by the first formula and C=e0*A/d2, which is the capacitance between two electrode points, which if lit with a light, would cast very ill defined shadows on each other.
As the meter hand is moved away from the PC board, increasing the distance between the dial's pads and the meter hand, the capacitance between the tip of the meter hand and the pad underneath becomes smaller. As the meter hand is moved away from the board, the meter hand sides cast more of a shadow on all of the other pads. Eventually, as the meter hand is moved away from the board, the capacitance formed becomes so diffuse that it is impossible to tell where the meter hand actually is pointing. This invention addresses these problems with solutions and provides a combination of elements that permits the use of a variable width electrode and a simplified receiver circuit employing a minimum of CMOS switches and multiplexer input/out ports.